1. Field of the Invention
This invention relates to a valve assembly and more particularly to an exhaust gas pressure responsive valve assembly.
2. Description of the Prior Art
The recirculation systems of exhaust gases have been developed as a method for reducing formation of oxides of nitrogen within exhaust gas during the combustion process in an internal combustion engine. These systems have valve means for controlling recirculation of exhaust gases in response to a vacuum signal produced responsive to the degree of opening of a throttle valve of a carburetor. Some of such prior systems further have an exhaust gas pressure responsive valve assembly which controls the above vacuum signal because it is desired to recirculate the exhaust gases at a rate proportional to the rate at which combustion air flows into the engine. These prior systems are disclosed, for example, in U.S. Pat. No. 3,834,366 and U.S. Pat. No. 3,802,402, wherein the above valve assemblies utilize exhaust back pressure, as signal pressure, to recirculate exhaust gases at a rate proportional to air flow. This means that a signal chamber of the valve assembly and conduits or pipes therefore are subject to exhaust gas which is high in temperature and exhibit acidic properties. At least a part of the exhaust gases transmitted to the signal chamber and conduits always remains even when it is desired to discharge the exhaust gas from the signal chamber or the like. Therefore, some considerations on materials of parts constituting the signal chamber or the like should be given to thereby reduce and prevent oxidation and corrosion of the above parts. Another consideration should also be given that the parts constituting valve portions or the like should be easy to manufacture because complex proceedings on some parts will be required.
However, prior valve assemblies have not taken into consideration the above mentioned problems so that there are drawbacks in that the first named-parts will be apt to be oxidized and corroded, and that the second named-parts will be quite difficult to manufacture.